1. Field of the Invention
The present invention relates to desmodromic valve and cam systems for traditionally configured internal combustion engines which utilize pushrods. In particular, the present invention relates to a valve train and cam system which eliminates the springs found in conventional valve trains by implementing a design which utilizes cam lobes having an internal follower groove in combination with hydraulic lifter follower assemblies. The present invention also relates to camshafts which have replaceable cam lobes providing various duration/lift adjustability options.
2. Background of the Invention
Most conventional internal combustion piston driven engines utilize valve trains to induct an air/fuel mixture into the cylinders and to expel the burned air/fuel mixture from the cylinders. Typically, each cylinder is assigned at least one intake poppet valve and at least one exhaust poppet valve. The valves are typically pushed down by rockers thereby opening the valve. To close the valve, that is to pull the valve back up so that it seats, most conventional valve trains utilize a spring which concentrically surrounds the valve stem. When the valve stem is pushed down by the rocker to open the valve, the spring is compressed. When the rocker lifts off from the distal tip of the valve stem, the valve then closes when the spring decompresses, thereby, pulling the valve stem up through the valve guides until the head of the valve seats in the valve seat.
The aforementioned conventionally configured valve train systems for opening and closing the valves has proven to be highly effective and reliable in the past. However, closing the valve by the force of the spring does have some disadvantages. Most notably, pushing the valves open against the force of the springs consumes engine power. The springs in an engine's valve train induce considerable tension into the valve train because they continuously force the valve mechanism against the rocker as the camshaft rotates. In other words, the valve springs are continuously pushing the valves closed. Another disadvantage is that because the cam mechanism cannot afford to have any “bounce” from the springs, the cam profile has to be somewhat gentle, i.e., it must gently push the valve. Another disadvantage is that when the motor is turned at high RPM's, the valves can “float” and hit the piston. Valve float happens when the speed of the engine is too great for the valve springs to handle. As a result, the valves will often stay open and/or “bounce” on their seats.
To overcome these disadvantages, innovative desmodromic valve trains have evolved over about the last century; however, in a very slow technological pace and in most applications with little or limited success. The term “desmodromic” arises from the two Greek words: “desmos” (controlled or linked), and “dromos” (course or track). A desmodromic system is also known as system that provides “positive valve actuation” wherein both strokes are “controlled”. In other words, desmodromic valves are those which are positively closed by a leverage system or follower, rather than relying on the more conventional springs to close the valves. Typically, a desmodromic valve operating system utilizes a camshaft that controls both the opening and closing of the valve.
Desmodromic valve trains have several advantages over conventional spring closed valves trains. A first major advantage is that in a desmodromic valve system there is almost no wasted energy in driving the valve train. In other words, the constant force that the springs exert on the valve train is removed. Another advantage is that because there is no tension and no possibility of “bounce” in the desmodromic system, the cam profiles can be as steep as the engine designer wishes them to be. This desirable aspect allows the engine to be more powerful. Thus, the manufacturer can use radical cam grinds or profiles for increasing performance. Another advantage is that when the motor is turned at high RPM's or even over-revved, the valves are still controlled, whereas when the valves are returned by springs the valves sometimes can “float” and hit the piston.
Nevertheless, even though desmodromic valve trains have the aforementioned advantages, they have had limited success in large scale commercial applications due to reliability issues, complexity of design, and valve train binding to name a few reasons. For instance, one of the major disadvantages of desmodromic valve trains is their sensitivity to change in size of the separate components of the system. In particular, the individual components (valves, cam lobes, rockers, etc.) of the valve train become enlarged at elevated temperatures because of thermal expansion of the metallic components. Also, the components of the valve train wear, thereby, decreasing the size of the components. As a cumulative result, of both cyclic expansion and contraction of the components caused by heating, and the shortening of components caused by wear, the tolerances of the valve train can change. The end results are components such as valves which do not seat properly, or unwanted binding in the valve train. Therefore, one of the major difficulties of prior art desmodromic valve train systems is the critical and accurate adjustment of various working components to ensure that the components operate together as intended without being subjected to binding, tension or excessive friction which results from the change of size in the individual components.
Another one of the problems with the aforementioned desmodromic valve train systems is that they have not been adapted to be installed or “retrofit” into existing modern conventional pushrod engines. That is to say, the aforementioned prior art desmodromic systems appear to utilize specialized designs which requires unique heads, valve trains and/or engine blocks. Thus, to use the aforementioned desmodromic systems, entirely new engine platforms have to be designed and built. Unfortunately, however, most manufacturers are not willing to invest the sizeable amounts of capital to build such desmodromic engines, or much less, market vehicles with engine platforms which have not been proven or which have had a past history for reliability problems.
Therefore, it would be advantageous to provide a reliable desmodromic valve and cam system that may be either integrated into a new engine design or of which may be retrofit onto an existing engine design without requiring the head or valves to be replaced. Furthermore, it would desirable to provide the aforementioned desmodromic system for a conventional pushrod engine, considering the popularity and success of the pushrod type platform. By providing a retrofit desmodromic system for pushrod engines, the cost of the upgrade could be maintained lower than that of a system which requires the entire head and valve train to be replaced. It would further be advantageous to provide a desmodromic valve and cam system which is simple and inexpensive to manufacture. Furthermore, it would be desirable to provide a desmodromic valve and cam system which would have interchangeable cam lobes such that the cam duration/lift could be adjusted. With such a feature, various cam lobes having varying profiles, durations, lift, etc. could be utilized on the same system by merely replacing the cam lobes. Such features would provide a wide array of adjustability in regards to being able to tune the engine's performance characteristics.